A mini PPA v2,my first portable attempt. - Page 2

Ok, now for some good news! All the parts arrived (finnally) and I assembled another amp. I changed 3 things when I built this amp:

1, the opamps I used on this one are opa1641s
2, used the pn4392 for Q3
3, used 1uf polyester caps for the C5 location.

Absolutely no oscillation, DC offset is 1mv or less! I am using my "test" headphones, a pair of beats solos, so I won't get into how it sounds, other than to say it does sound very promising. I think got the other amp I will replace the C5's, and possibly change the opa637's out for 627 units. I have one set of PCBs left, think that will be used for the AD8610 opamps I have. Pics comming soon. Now all I have to do is built the faceplate for the Hammond case and I will have a truly pocketable PPA!

A few pics of the assembly

Sorry for the crappy cell phone pics......

 

It should be better than the 627/37 combo, but not a whole lot. The specs on the "1641" amps (can't use a dual op amp 1642 with this design) say the current draw of these amps is ~1.8ma, that is quite a bit better than the 627/37 combo

 

 

It depends on where i set the biasing, right now I have the buffers biased at 15ma, and the op-amps biased at 1ma, nominal current draw on the amp (just power on, no music playing) with these settings is right at 101ma. Playing music through the (crappy I know...) solo-HD headphones is about 110ma. So with the 900mah battery, I can expect at least 8 hours of play time.

 

Of course I could increase or decrease the biasing, Tangent states on his PPA-v2 site that 10ma buffer bias also worked fine, just have to play with it
 

Ha, Ha. Yes, I am working inside a truck! in fact i built this whole amp in my work truck. The reason is that i am an electronics tech, and at times i have to work out in the field.

 

Yes, there is a battery inside the enclosure, it is a 900ma 3 cell li-po pack with a protection PCB, i purchased the cells from all-battery(dot)com, part number 463455, they are; 4.6mm thick, 34mm wide and 55mm long, hence the 46-34-55 part number.

 

As for the layout even i was surprised that i could get it all to fit. I have already revamped the board though, and the layout is much better, and i gained a little board space to boot. The one drawback (and big improvement) to the new layout, is that it places SMD caps very close to the op-amp power pins on the backside of the op-amp daughter card. The caps are pricey, and mouser had 7 of them  505-PET28-1/63/10  , i bought 6, so there is one left!

 

I am trying to find an alternative, I want to keep the SMD cap because it allows me to place the cap very, very close to the op-amp, this is a good thing.

 

as I stated I am working out in the field, so getting critical listening time in is very hard to do. I did listen to a few songs last night with My M50's, I was very tired, but I liked what i heard, a lot!

 

Next big project for this amp will be to figure out a way to socket the op-amp board, i have been looking for a solution to this since i started this amp. Mill-Max makes some nice sockets that are low profile an designed to press into the PCB, the problem is that they need a machined round pin (.012-.018 or so dia.) to work well. They don't offer a right angle male pin that will work. because the op-amps pretty much have to be SOIC8 packages, rolling op-amps is a huge ordeal. My new design however, if I can get some sort of socket arraignment, would allow the individual op-amp boards to be swapped. So it would be akin mounting the op-amp to a brown dog adapter, only with the feedback and gain resistors and caps on the board, and just swapping out the assembly.

 

And I want to order up some OPA827's to try, I keep hearing good things about these op-amps and they have pretty good voltage specs for this amp. I do like the 1641's I hope to get more ear time on them but for the few hours i have listened to them i haven't found anything i dislike about them.

 

Joe

I'll try to post moar pics tonight, slow internet out here in the hills.

 

I did find a good selection of SMD film caps at digikey, however most are not polyester, but Polyethylene (PEN), Not sure where these stack up in the world of audio caps. Anyone ever tried them?

one other note, with the 10K alps volume pot, the "default" 10X gain is a bit much, with either the solo's or the M50's, 50% volume is ear-splittingly loud, with my custom IEM's 25% volume is all i can take. I think for the next build I will shoot for 5-8x gain.